#!/usr/bin/env perl -l

use strict;
use warnings;

my $line = 'EntityMappingFetchByName?[](EntityName$ = DERIVED_ATTRIBUTE_TABLE, FieldNames$[] = [ USER_ENTITY_NAME ], text${} = { this is a test }), line 6';

my $re = qr< ( { [^}]+ } | \[ [^\]]+ \] ) >x;

print "MATCH = $1" while $line =~ /$re/g;